A system undergoes a reversible adiabatic process. The entropy of the system

In general, the temperature of the system may change during a process. If the process is reversible, the change in entropy is defined as

$S_f-S_i={\int }_i^f\frac{∆Q}{T}$

In an adiabatic reversible process, no heat is given to the system. Therefore, entropy of the system remains constant.